For production of fresh water from seawater or for production of clean water from river or lake water, reverse osmosis membrane apparatuses having a reverse osmosis membrane module, for example, are used. By using a reverse osmosis membrane apparatus, a to-be-treated water (clarified water) produced by subjecting e.g. sea, river or lake water as raw water to pretreatment such as sterilization treatment of adding a fungicide into the sea, river or lake water or a treatment of removing impurities by using e.g. a sand filter, is pressurized to have a pressure of about 6.0 MPa by a high pressure pump, for example, and is supplied to a reverse osmosis membrane module, and the to-be-treated water is separated by using a reverse osmosis membrane provided in the reverse osmosis membrane module to obtain permeate water to be production water.
A reverse osmosis membrane module is composed of a high pressure vessel and a plurality of reverse osmosis membrane elements arranged in series in the high pressure vessel. Patent Documents 1 and 2 disclose spiral-type reverse osmosis membrane elements. A spiral-type reverse osmosis membrane element has a cylinder shape and has a structure where a sac-like reverse osmosis membrane having a flow path material therein is spirally wound via a mesh spacer around a center pipe in which permeate water is collected, and a brine seas is provided at an end of the outer peripheral surface. With the reverse osmosis membrane module, to-be-treated water is separated with reverse osmosis membrane elements sequentially from one of the first stage into permeate water and concentrated water containing saline matters and impurities to obtain permeate water by each of the reverse osmosis membrane elements, and the concentrated water separated from the permeate water is further separated into permeate water and concentrated water by the reverse osmosis membrane element of a later stage. Therefore, the salinity and concentration of impurities of concentrated water are higher at relatively downstream side.
FIG. 6 shows an example of a construction of a reverse osmosis membrane module having a high pressure vessel and spiral-type reverse osmosis membrane elements arranged in series therein, which is disclosed in Patent Document 1. In FIG. 6, the reverse osmosis membrane module 100 comprises a high pressure vessel 102 and four to eight reverse osmosis membrane elements 104 arranged in series in the high pressure vessel 102. To-be-treated water tw is supplied at a high pressure to an inlet opening 102a of the high pressure vessel 102 by a high pressure pump (not shown) provided in a to-be-treated water supply passage 114. The to-be-treated water enters into the reverse osmosis membrane element of the first stage from the inlet, and it is separated into permeate water pw and concentrated water cw in the reverse osmosis membrane element 104.
The permeate water pw is flown into the center pipe 106, and the concentrated water cw is flown out from the outlet of the reverse osmosis membrane element 104. In the reverse osmosis membrane element 104 of the first stage, the inlet of the center pipe 106 is obstructed with an end cap 108. The center pipe 106 of each of the reverse osmosis membrane elements 104 is connected with a connector 110. Accordingly, the permeate water pw from each of the reverse osmosis membrane elements 104 joins together and is discharged from an outlet opening 102b of the high pressure vessel 102 to a permeate water discharging passage 116.
As the interior of the high pressure vessel 102 is partitioned with brine seals 112 provided on outer peripheral surface of each of the reverse osmosis membrane elements 104, the concentrated water cw flown out of each of the reverse osmosis membrane elements 104 is flown into the reverse osmosis membrane element of a later stage as to-be-treated water without going past the reverse osmosis membrane element of the later stage. The concentrated water cw is thereby permeated sequentially with the reverse osmosis membrane elements. The concentrated water cw discharged from the reverse osmosis membrane element 104 in the last stage is discharged from an outlet opening 102c formed at the outlet end of the high pressure vessel 102 to a concentrated water discharging passage 118.
In general, in reverse osmosis membrane apparatuses applied to seawater desalination plants, in case of change in temperature of seawater or change in water quality, the amount of the permeate water is controlled via a control valve provided in the to-be-treated water supply passage at the outlet side of the high pressure pump or via rotation speed of the high pressure pump. The control valve needs to be manufactured from a seawater-resistant material such as super duplex stainless steel and needs to have resistance against high pressure, and thus it is required to have a large thickness, which may result in high cost.
The water quality of the permeate water may be declined along with aging deterioration of the reverse osmosis membranes or increase in the seawater temperature, thereby not to satisfy a target value. In addition, the water quality of the permeate water varies depending upon the water quality of the raw seawater which is source of the clarified seawater subjected to pretreatment. In order to obtain a targeted water quality, it is effective to change the recovery rate (amount of permeate water/supply amount of clarified seawater) of the reverse osmosis membrane. However, in this case, the amount of permeate water may change, and the permeate water may not be supplied stably. As a method for increasing the amount of the permeate water, there is a method of increasing the supply pressure of the clarified seawater supplied to the reverse osmosis membrane element. However, as the pressure capacities of the reverse osmosis membranes and the high pressure vessel have limitations, water quality improvement by increasing the supply pressure is limited.
Patent Documents 1 and 2 disclose, as a measure for improving water quality of the permeate water, a method of returning a part of the permeate water to the to-be-treated water, mixing the returned permeate water with the clarified seawater, and supplying the returned permeate water again with the clarified water to the reverse osmosis membrane module. That is, in a plurality of reverse osmosis membrane elements arrange in series, as the to-be-treated water is passed through the reverse osmosis membrane elements sequentially from ones of an earlier stage to ones of a later stage, the concentration of the concentrated water becomes higher as the stage of the reverse osmosis membrane element is later. Accordingly, the water quality of the permeate water separated by the reverse osmosis membrane element of a later stage is worth than that of the permeate water separated by the reverse osmosis membrane element of an earlier stage. Therefore, in order to improve the water quality, the permeate water passed through the reverse osmosis membrane element of the later stage is returned to the to-be-treated water to reduce the salinity of the to-be-treated water to be supplied to the reverse osmosis membrane elements, or a permeate water from the reverse osmosis membrane element of the earlier stage, which has a relatively good water quality, is obtained.